The present invention relates to a device for detecting the residual capacity of a battery, especially, a lithium secondary battery.
With the recent development of small size and portable equipment, secondary batteries such as nickel-cadmium batteries have come to be used for power supplies for radio-cassette recorders and portable VTRs. As long as an A.C. power source is used, there is no fear of exhaustion of the power supply, but when a battery is used as the power source, the capacity of the battery is limited and, hence, depending on the apparatus used, it becomes very important to know how long the apparatus can further be used. In such case, if there is provided a device to display the residual or used capacity of the battery, it becomes easy to presume how long the residual operating time. This would offer much convenience to the user.
However, the discharge curve of a nickel-cadmium cell today is flat as shown in FIG. 7 by curve I and, hence, it is very difficult to detect its residual capacity. Since the discharge curve of a lead cell is also relatively flat as shown by curve II in FIG. 7, detection of the residual capacity is also difficult, though it is not so difficult as with the nickel-cadmium cell.
In contrast, in the case of a lithium secondary cell, its discharge curve is sloped as shown by curve III in FIG. 7 and, hence, it becomes possible to detect the residual capacity of the cell from the discharge curve. Conventionally, checking of the consumed state of a lithium secondary battery has been performed, as is also described in Japanese Laid-Open Patent Publication No. Hei 1-100465 and No. Hei 1-100479, with the actual load connected thereto when performed in the loaded state, and with a dummy resistor corresponding to the actual load connected when performed in the unloaded state. Then, in the case of a lithium secondary battery with a video camera connected thereto as the actual load, the load is of the order of several watts and, hence, in the measurement under a no load condition, a corresponding dummy resistor of several watts becomes necessary. Besides, since the discharge curve of the lithium secondary battery is such that the discharge voltage greatly varies with the discharge load, accurate detection of the residual capacity becomes difficult. This is because, for example, when a lithium battery is used as the power source of an 8 mm video camera, the load greatly changes according to how its zoom mechanism is operated or stopped, and the display of the residual capacity is affected by the change.
Further, a large load connected to the battery as the dummy resistor for checking the residual capacity of the battery consumes power wastefully. This cannot be reasonable and even leads to such great disadvantages that heat is produced and the device becomes bulky.